Oil wells typically comprise one or more concentric metal pipes in a bore hole through an earth formation. Usually, the diameter of the borehole decreases with distance from the top of the borehole and the number of concentric pipes decreases accordingly. Cement is provided between the outermost pipe and the earth formation outside the bore hole, and usually also between one or more pairs of radially successive pipes, although there may also be pairs of pipes without cement between them. The combination of one or more pipes and cement is referred to as a well bore.
The cement is used to prevent leakage through the well bore, for example from a deeper layer to the sea floor. Flaws in the cement can create a risk of leakage from the well bore. Two types of flaws can be distinguished: bond index loss and the presence of a micro annulus. The bond index is a measure of the fraction of space between successive pipes or between the outermost pipe and the surrounding formation that is filled by cement. Usually, if the bond index is less than a pre-determined threshold continuously over an interval of distance from the top of the well bore and the interval exceeds a predetermined length, this is considered to be an indication of a serious risk of leakage. A micro annulus is a lack of bonding of the cement to the pipe, which can also be a serious risk even if it does not present a dangerously low bond index.
It is known to use ultrasound pulse reflection measurements to test well bores for the presence of such flaws in the cement. A probe is lowered into the well bore through the inner pipe and ultrasound pulses are transmitted from the probe. The time of arrival and amplitude of the pulse is detected at a receiver. When measurements of the time of arrival or amplitude obtained show changes from a nominal measured time of arrival and amplitude when the probe is lowered, this is taken as an indication of the presence of flaws.
In practice, transmission of a pulse in a well bore does not result only in a single received pulse that is a copy of the transmitted pulse. The received pulse starts with a clearly distinguished delay after transmission, but it lasts much longer than the transmitted pulse. This is due to the availability of different transmission paths and wave modes in the well bore. The start of the reflected pulse can be associated uniquely with the fastest mode along the shortest path, but the later part of the received pulse is due to a mix of modes and paths. The signal components due to these modes and paths depend on the configuration of the well bore which are unrelated to flaws, and also on variations of intrinsic material properties or changes in the surrounding formation.
WO9935490 discloses ultrasonic inspection of a well bore. A transmitter and a receiver axially offset to the transmitter are used. A pulse is transmitted at an angle to the inner surface of the inner pipe of the well bore, so as to excite waves that travel axially through pipe. WO03083466, which refers to WO9935490, discloses use of a phased array of ultrasonic transducers to provide for transmission in a focused beam in the well bore without need for mechanical rotation. As disclosed in WO9935490, inhomogeneity in the cement, presence of fluid etc can give rise to wave scattering. WO9935490 relies on the fact that the cement-formation interface echoes depend on propagation through the cement. Their time delays depend on wave speed in the cement and cement thickness. Echo amplitudes depend on wave decay rate. If the echoes due to reflection from the cement formation interface can be identified, cement properties can be derived from the echoes and arrival time of echoes from scatterers can give information about the location of scatterers and the amplitude can give information about the size.
WO9935490 describes a wide variety of analysis techniques that can be used to extract information about the cement. The early arriving echo can be used to evaluate a pipe of the well bore for corrosion and perforations, or the presence of gas-like material at the first pipe-cement interface. Wave dispersion characteristics can be used to determine pipe thickness.
It is more difficult to obtain information from later arriving echoes that arrive after these early-arriving echoes, unless the later arriving echoes can be attributed to individual modes. WO9935490 describes processing of later arriving echoes to determine multiplicity for qualitative determination of cement strength, to determine propagation time inside the cement; and determination of whether echoes arose from scatterers in the cement or at the cement-formation interface. An inversion method is mentioned wherein the amplitude reduction of early-arriving echoes is used to estimate the beam profile that has been transmitted from the pipe into the cement, and later arriving echoes are used in conjunction with this beam profile to extract scattering information from the cement-formation echoes. However, this approach is useful only for later arriving echoes that can be distinguished.
It may be difficult to extract ultrasound information that is relevant for well bore inspection. Ultrasound inspection works best to provide complete results when the well bore contains only a single metal pipe between the inner space and the formation around the well bore. A well bore with a plurality of concentric pipes may have additional variability, such as changes in the number of concentric pipes, pipe diameters, pipe thickness, pipe material properties well bore diameter, cement properties, eccentricity of the pipes, well bore diameter, cement and fluid properties etc with distance from the top of the well bore, which give rise to ultrasound propagation changes that are unrelated to flaws in the cement. Moreover, it becomes difficult to interpret waves that may have penetrated beyond the first pipe-cement interface. Waves that leave the inner pipe outwardly are mainly reflected before they reach the interface between the cement and the surrounding rock formation, particularly at usual frequencies in the range above one hundred kilohertz. Better penetration can be achieved by using relatively low frequency ultrasound, with wavelengths larger than the distance between the pipes and the surrounding formation. But at these frequencies later arriving echoes are even more difficult to distinguish, and the well bore configuration still causes variations unrelated to flaws. The prior art provides only limited ways of extracting ultrasound information that is relevant for well bore inspection.